Korean Utility Model Registration No. 175040 filed by the inventor discloses an example of the above device, which is shown in FIGS. 1 and 2.
FIG. 1 is a perspective view illustrating a spine fixing screw assembly disclosed in the above document, and FIG. 2 is a sectional view illustrating the assembled posture of the screw shown in FIG. 1.
As shown in FIGS. 1 and 2, the spine fixing screw assembly 1 has vertical extensions 7 at both sides of a rod receiver 5 for receiving a rod 3. Each of the vertical extensions 7 has a female screw portion 11 for transporting a male screw 9 for fixing the rod 3 received in the rod receiver 5. The spine fixing screw assembly 1 is so configured to transport the male screw 9 via connector members 13 and wrap a rim 15 around the vertical extensions 7.
In the above configuration, the connector members 13 connect between the lower end of the rim 15 and a recess 17 in a lower portion of the male screw 9. The undescribed reference numeral 2 designates a screw, and the reference numeral 10 designates a groove for a wrench.
The spine fixing screw assembly 1 configured as above has the following problems:
First, since threads of the male screw 9 primarily contact with those of the female screws 11 when the male screw 9 is initially engaged into the female screws 11, the central axis of the male screw 9 is hardly aligned with the central axis of the vertical extensions 7 thereby probably prolong the operation time.
Second, the recess 17 is formed adjacent to the lower portion of the male screw 9 so that the male screw 9 is restricted in the number of threads to lower the engaging force.
Third, since the connector members 13 are formed in a lower portion of the rim 15, the connector members 13 primarily contact with the rod 3 if the rod 3 is flexed. Then, the male screw 9 does not sufficiently contact with the rod 3 thereby potentially dropping the fixing force.
Fourth, since the male screw 9 has a flat lower end, if the rod 3 is settled in the rod receiver 5 in a flexed or tilted manner, the contact area is reduced to lower the fixing force of the rod 3.
Fifth, since the rod receiver 5 has a horizontal settling face 6, the rod 3 is not closely contacted with the rod receiver 5 if arranged in a tilted manner in respect to the settling face 6 so as to potentially drop the fixing force thereof.
Sixth, the rim 15 is not further transported downward from the position of fitting thresholds formed in the outer periphery of the vertical extensions 7. In an operation where the rod is necessarily fixed more strongly, the male screw 9 is necessarily subjected to a very strong engaging force. Since the rim 15 has a uniform thickness, the rim 15 may be distorted into an oval shape or radially fractured since an excessive force may be applied to the rim 15 in engagement of the male thread 9.